Welcome to the board.
I doubt the tuna is contaminated with wheat from a crop rotation. Gluten is the protein in the seed of wheat which would not get into the soy. Any wheat missed in the harvest would just be a seed on the ground and not harvested from the soy plant. I doubt they would let any wheat germinate either so it is most likely not growing there.
... My guess is it is the soy that was bothering you, or maybe the tuna?
Perhaps try tuna packed in water instead. Then there will be no chance of contamination at all.